Valve control device

ABSTRACT

The invention relates to a device for displacing a moving part along a longitudinal axis relative to a housing, said device being symmetrical about a plane extending longitudinally relative to the moving part, the device comprising two structures that are secured to said moving part and elements of active material comprising at least two blocks extending between inside walls of each structure, with the elongation directions of the blocks being antisymmetrical about an axis substantially perpendicular to the displacement axis of the moving part, wherein the structures are flexible and suitable for displacing the moving part by deforming as a function of the elongation of the blocks.

GENERAL TECHNICAL FIELD

The present invention relates to devices for controlling hydraulicvalves or slides, in particular in hydraulic circuits, e.g. in theaviation or automotive fields.

More precisely, the invention relates specifically to devices forcontrolling hydraulic circuit valves enabling the valve to moveprogressively, and providing greater safety in use over a wide range oftemperatures.

STATE OF THE ART

Devices for controlling valves in hydraulic circuits are already knownin the state of the art.

Such control devices are used in numerous systems, for example in brakeservo-control systems, or in flight control actuators in the field ofaviation.

Certain devices in the state of the art are intended to adjust thehydraulic flow rate of the circuit to a desired level.

In general, hydraulic flow rate is controlled by moving a needledisposed in an orifice, the position of the needle in the orifice andthe displacement thereof serving to regulate the flow rate of theliquid.

The control systems most commonly in use at present are electromagnetic.They are large in volume and they consume a large amount of energy.

Certain valve control systems make use of piezo-electric actuators formoving the needle.

However, those devices do not give full satisfaction.

The needle is moved along the elongation axis of the piezoelectricmaterial. In that type of configuration, a change in the volume of thepiezoelectric material as a function of temperature leads to instabilityin the position of the needle when the temperature varies over a largerange. Furthermore, most systems including such a piezoelectric devicepresent displacement of the operating zero position that is greater thanthe operating range.

SUMMARY OF THE INVENTION

The invention seeks to mitigate those drawbacks.

An object of the invention is to propose a device which presents greatstability over a large range of temperatures, typically −55° C. to +80°C.

Another object of the invention is to propose a system enabling valvesto be actuated with a force that is typically equal to 50 newtons (N),over a displacement amplitude that is typically of +0.5 millimeters(mm).

Finally, an object of the invention is to propose a valve control devicethat is compact.

To this end, the invention provides a device for displacing a movingpart along a longitudinal axis relative to a housing, said device beingsymmetrical about a plane extending longitudinally relative to themoving part, the device comprising two structures that are secured tosaid moving part and elements of active material comprising at least twoblocks extending between inside walls of each structure, with theelongation directions of the blocks being antisymmetrical about an axissubstantially perpendicular to the displacement axis of the moving part,wherein the structures are flexible and suitable for displacing themoving part by deforming as a function of the elongation of the blocks.

The invention advantageously further comprises the followingcharacteristics taken singly or in any technically feasible combination:

-   -   each structure comprises a plate extending substantially        parallel to the displacement axis of the moving part and oblique        pushers integral with the plate, at least one pair of pushers        being secured to the moving part at their ends via at least one        junction block;    -   each structure includes portions of narrow section between the        plate and the oblique pushers, and between the oblique pushers        and the junction blocks connected to the moving part, these        portions of narrow section being suitable for imparting        flexibility to each structure;    -   each structure comprises a plate extending substantially        parallel to the displacement axis of the moving part and oblique        pushers fitted to said plate, at least one pair of pushers being        secured to the moving part at their ends via at least one        junction block, the connection between the plate and the pushers        and/or the connection between the pushers and the junction block        being a resilient hinge without play constituted by at least one        connecting blade;    -   the device includes means suitable for prestressing the        displacement means between the plates;    -   the pushers are geometrically antisymmetrical relative to a        plane perpendicular to the plates;    -   one junction block is connected to the moving part and the other        junction block is fixed relative to the housing external to the        structures, the fixed block being secured to a rod coming into        abutment against a screw for adjusting the displacement zero        position of the moving part;    -   the pushers are geometrically symmetrical about a plane        perpendicular to the plates;    -   both junction blocks are connected to the moving part;    -   the device includes at least one diaphragm on either side of the        junction blocks, the diaphragms being flexible along the        displacement axis of the moving part and stiff transversely        relative thereto;    -   the diaphragms are suitable for providing sealing between the        moving part and the housing;    -   the housing is filled with a counterpressure liquid;    -   at least one diaphragm includes bellows;    -   the device includes at least one expansion chamber for the        filler liquid; and    -   each expansion chamber includes a cutout constituting a portion        of the wall of the housing, said cutout being connected to the        housing by bellows;

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics, objects, and advantages of the invention appearfrom the following description which is purely illustrative andnon-limiting, and which should be read with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic perspective view of one possible embodiment ofa valve control device of the invention;

FIG. 2 is a diagrammatic longitudinal section view through a possibleembodiment of a valve displacement device, said device being locatedinside the device shown in FIG. 1;

FIG. 3 is a diagrammatic longitudinal section view of a possibleembodiment of a valve control device including a filling liquid insideits housing, the device having means allowing the filling liquid toexpand thermally;

FIG. 4 a is a diagrammatic perspective view of a first possibleembodiment of a structure for actuating the moving part;

FIG. 4 b is a diagrammatic perspective view of a second possibleembodiment of a structure for actuating the moving part;

FIG. 5 is a diagrammatic longitudinal section view of another possibleembodiment of a device for displacing valves, this device being placedinside the device shown in FIG. 1;

FIGS. 6 a and 6 b are diagrams of an embodiment of a diaphragm fixed ona device as shown in FIG. 5; and

FIGS. 7 a to 7 c show various operating states of a FIG. 5 device.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a possible embodiment of a valve control device 1comprising an outer housing 2 in which there is disposed a rod 3extending along a substantially longitudinal axis of the housing 2.

The housing 2 may be of any shape, but it is preferably in the form of arectangular parallelepiped.

The rod 3 is movable in translation along the longitudinal axis of thehousing 2. At least one of its ends is connected to a valve in ahydraulic circuit. Double-headed arrows visible in FIG. 1 representdisplacement of the rod 3 relative to the housing 2.

Sealing is provided between the housing 2 and the rod 3 by means 4suitable for securing the rod 3 and the housing 2 together in such amanner as to guarantee leak-tightness while nevertheless ensuring thatthe rod 3 can be displaced relative to the housing 2. By way of example,the sealing means comprise a diaphragm 4 closely surrounding the rod 3and also connected to the housing 2. The diaphragm 4 is flexible andallows the rod 3 to be displaced longitudinally in the housing 2.

When the valve is opened or closed to a greater or lesser extent, fluidflows around the housing 2, for example.

Means for actuating the rod 3 are contained within the housing 2.

FIG. 2 is a diagram of a first possible embodiment of means foractuating the rod 3. The actuator means are symmetrical about a plane 30containing the longitudinal axis of the rod 3. Consequently, this figureshows only the top half of the actuator means, above the plane 30. Theentire system is shown in longitudinal section in FIG. 3. FIGS. 4 a and4 b show a portion of the actuator system in a perspective view.

With reference to FIGS. 2 to 4 b, the actuator means comprise means 5for causing the rod 3 to slide longitudinally. By way of example, themeans 5 comprise a part that is substantially in the form of a thickplate 5 constituting a rectangular parallelepiped and pierced by a holesurrounding the rod 3.

The rod 3 may equally well be circular or square in section.

The means 5 are fixed relative to the housing 2 and are fixed to itsinside walls. By way of example, the part 5 is fixed relative to theinside wall of the housing 2 via one of its sides that are parallel tothe axis of the rod 3.

Means for displacing the rod 3 enable it to be caused to slide in themeans 5.

In the diagram of FIG. 2, the displacement means comprise means 6 fordeforming a structure 7 secured to the rod 3. Naturally, the device 1has two structures 7 on opposite sides of the plane 30.

In FIG. 2, the top structure 7 is substantially in the form of a bridgesurrounding the central portion of the rod 3, the ends of the rod 3projecting from the structure 7 and the housing 2 in order to actuatevalves.

The structure 7 comprises a top plate 10 extending in a rest positionsubstantially parallel to and spaced apart from the axes of the rod 3and the part 5. The top plate 10 is substantially in the form of arectangular parallelepiped extending substantially parallel to the topsurface of the part 5. For greater clarity, this top plate 10 is notshown in FIGS. 4 a and 4 b.

The structure 7 also has lateral pushers 9 made integrally with the topplate 10 and extending obliquely towards the rod 3. The ends 13 of thepushers 9 are secured to the rod 3 and form connection blocks, forexample. FIGS. 4 a and 4 b show two possible embodiments of the pushers9. In FIG. 4 a, the right section of the pushers tapers going from theplate 10 towards the blocks 13. In FIG. 4 b, the right section of thepushers 9 is constant and square in shape. Other embodiments are alsopossible. For example, the dimensions of the plate 10 and of the pushers9 along an axis perpendicular to FIGS. 2 and 3 can be the same.Dimensioning depends on the section of the rod 3.

The structure 7 has portions 8 of narrow section at the junctionsbetween the top plate 10 and the oblique pushers 9, and also at thejunctions between the oblique pushers 9 and the blocks 13. The portionsof narrow section 8 thus form locations in the part 7 that are highlyflexible, thereby forming hinge type means.

The deformation means 6 deform the structure 7 which in turn moves therod 3 in the means 5.

With reference to FIG. 2, the means 6 comprise blocks of activematerials 6 a and 6 b connected firstly to the outside surface of themeans 5 and secondly to the inside face of the top plate 10 of thestructure 7. The blocks 6 a and 6 b may equally well be circularlysymmetrical in radial section or they may be square. Their ends bearagainst two plane surfaces. The first plane surface is the bottom wallof the top plate 10. The second plane surface is the top wall of themeans 5.

Prestress means 11 enable the deformation means 6 to be prestressedagainst the parts 5 and 7. For example, the prestress means comprise athreaded rod 11 co-operating with a nut 12 and a washer 14. The nut 12presses the washer 14 against the outside face of the top plate 10 inorder to prestress the parts 7, 6, and 5 against one another.

The active materials 6 a and 6 b may be of the electrostrictive,magnetostrictive, or piezoelectric type. They are advantageously made ofpiezoelectric materials.

There follows a description of the mechanism for deforming the structure7 and displacing the rod 3.

The elongation axis of the active materials 6 a and 6 b is normal to theaxes of the rod 3 and the hollow part 5. Similarly, the elongation axisof the active materials 6 a and 6 b is normal to the top plate 10 of thestructure 7.

In FIG. 2, arrows 22 show the elongation directions of the activematerials 6 a and 6 b during a first stage of applying an electricalvoltage.

The arrows 22 thus show that during the first stage, the active material6 a elongates positively, whereas during the same stage, the activematerial 6 b does not elongate or is subjected to negative elongation.

This difference in elongation causes the top plate 10 to pivot in thedirection indicated by arrows 21. This pivoting is made possible by theflexibility of the structure 7 due to the hinges 8.

The pivoting of the top plate 10 gives rise in turn to displacement ofthe oblique pushers 9.

Since the ends 13 of the oblique pushers 9 are secured to the rod 3,positive elongation of the material 6 a causes the rod 3 to move in thedirection indicated by arrow 20.

This actuates displacement of a valve in a hydraulic circuit located atthe end of the rod 3.

It will readily be understood that it is possible to reverse thedirection in which voltage is applied to the active materials 6 a and 6b. It is then possible to obtain positive elongation of the material 6 band negative elongation of the material 6 a. It will be understood thatthe top plate then pivots in the direction opposite to arrows 21, andthat the rod 3 moves in the direction opposite to that given by arrow20.

The displacement of the valve at the end of the rod 3 will therefore bereversed relative to the first voltage-application stage and hydraulicflow will thus be modulated.

Displacement of the rod 3 is easily modulated by the elongationamplitude of the materials 6 a and 6 b, thus enabling the hydraulic flowrate in the circuit to be modulated accurately.

Advantageously, the active materials 6 a and 6 b and the voltagesapplied in the various stages enable the rod 3 to move over a strokehaving an amplitude of ±0.5 mm.

Advantageously, the force exerted at the end of the rod 3 is equal to 50N.

The fact that the active materials 6 a and 6 b elongate inantisymmetrical directions relative to a longitudinal axis 31 of the rod11 makes it possible to obtain automatic compensation of the neutral orequilibrium position. This compensation takes place regardless oftemperature.

For zero applied voltage, the rod 3 naturally returns to a stablecentral position. Thus, this system provides good stability for theneutral position.

The deformable structure 7 enables the elongation of the activematerials 6 a and 6 b to be amplified greatly.

The housing 2 is placed inside a hydraulic circuit. The static pressureof the liquid surrounding the housing 2 may therefore be very high.Consequently, the operation of the diaphragm 4 might be affected ifthere is nothing inside the housing 2 to counteract said pressure.

Thus, in order to counter the large static pressure of the liquidsurrounding the housing 2, FIG. 3 shows that in a variant of theembodiment shown in FIG. 2, the inside of the housing 2 isadvantageously filled with a counterpressure liquid 18, this liquidbeing non-aggressive for the means for actuating the rod 3.

When the inside of the housing 2 is filled with a liquid 18, and inorder to accommodate the very large thermal expansion of the fillingliquid 18 relative to the walls of the housing 2, FIG. 3 shows thatbellows 15 are provided at at least one diaphragm 4 in order to enablethe filler liquid to expand.

In a variant, at least one expansion chamber is provided for the fillerliquid 18. By way of example, the expansion chamber comprises a cutout17 in a portion of the wall of the housing 2, this cutout beingconnected to the housing 2 by a bellows 16. Thermal expansion of thefiller liquid 18 is thus accommodated by moving the part 17.

The bellows 15 of the diaphragms 14 are optionally associated with oneor more expansion chambers.

Preferred Embodiment

FIG. 5 is a diagram of a second possible embodiment of the means foractuating the rod 3.

In this figure, elements similar to those of FIGS. 2 to 4 b are givenidentical numerical references.

As for the first embodiment, the actuator means are symmetrical aboutthe plane 30 containing the longitudinal axis of the rod 3.

However, in the second embodiment, the actuator rod 3 is no longerplaced inside an outer housing 2, and the housing 2 no longer hasinternal sliding means. One of the ends of the rod 3 is situated on theleft of the device shown in FIG. 5.

As shown in FIG. 5, the displacement means comprises means 6 fordeforming two structures 7 extending on opposite sides of the plane 30and perpendicularly to the plates 10 of the structures 7, i.e.substantially parallel to a prestress element 11 as described in greaterdetail below.

The means 6 comprise at least two blocks of active material 6 a and 6 b.Each block 6 a or 6 b may comprise a pair of blocks stacked one on theother.

The active materials 6 a and 6 b may be of the electrostrictive,magnetostrictive, or piezoelectric type. They are advantageously made ofpiezoelectric materials. The blocks 6 a and 6 b are biased in thedirection of the major dimension.

Each structure 7 has a top plate 10 which, in the rest position, extendssubstantially parallel to and at a distance from the plane 30. Eachstructure 7 also has lateral pushers 9 made integrally with the topplate 10 and extending obliquely towards the plane 30.

The pushers 9 may alternatively be separate pieces fitted to the plates10 via one or more flexible metal blades, or by blades made of amaterial other than metal, the blades serving to provide resilient linksbetween the pushers 9 and the associated plates 10.

The pushers 9 are geometrically antisymmetrical about a longitudinalplane 30 containing the axis of the prestressed means 11, the plane 31being perpendicular to the plane of FIG. 5 and to the plate 10. Thus,starting from the plates 10, the pushers extend towards the rod 3, fromright to left for the device as shown in FIG. 5.

As in the first embodiment, the prestress means preferably comprise athreaded rod 11 co-operating with a nut 12 and a washer 14. The nut 12presses the washer 14 against the outside face of the top plate 10 so asto prestress the parts 7 and 6 against each other. Compressing theblocks 6 a and 6 b avoids them suffering damage. However, the prestressmeans are sufficiently flexible to avoid impeding movement of the plates10.

The structures 7 which are symmetrical relative to each other aresecured to each other by blocks 13 situated at one end of each pusher 9.

The pushers 9 are secured to each other so as to form junction blocks13.

Because of the symmetry about the plane 30 and the antisymmetry aboutthe plane 31, each pair of pushers 9 forms an angle that opens out inthe same direction, to the right in FIG. 5, but which may be of valuethat differs depending on torque, as described below.

Each structure 7 has portions 8 of narrow section at the junctionsbetween the top plate 10 and the oblique pushers 9 and between theoblique pushers 9 and the blocks 13. The portions 8 of narrow sectionthus form locations of great flexibility in the structure 7, therebyforming hinge type means.

When the pushers 9 are fitted to the plate 10, the hinge 8 is formed bya link blade between the pusher 9 and the plate 10.

It is also possible to provide for the pushers 9 to be connected to thejunction blocks 13 via at least one respective flexible blade made ofmetal, or made of a material other than metal, the blade providing theresilient connection between each pusher 9 and the corresponding blocks13.

It can thus be understood that the flexible blades perform the samefunction as the portions of narrow section in the pushers 9, formingresilient hinges without play, that give flexibility to each structure7.

The connection blocks 13 are both connected to link means leading toother mechanical parts that pass through diaphragms 4 situated to theright and to the left of the structures 7.

The block 13 a is movable longitudinally in the plane 30 and isconnected to a connection rod 55 which passes through the diaphragm 4 a.The rod 55 is itself connected to the valve control rod 3.

The block 13 b is fixed relative to the housing 2 while the device is inuse. It is connected to a rod 56 situated on the axis of the rod 55 andon the axis of the rod 3 in the plane 30. The rod 56 passes through thediaphragm 4 b.

At its end remote from the block 13 b, the rod 56 has a head 57. Thehead 57 comes into abutment against the end of the threaded shank 58 ofa screw 60. The head 59 of the screw 60 is situated outside the housing2. The screw 60 enables the displacement zero of the rod 3 to be set.This zero setting of the displacement of the rod is performed by movingthe structures 7 in translation parallel to the plane 30. The structures7 float relative to the housing 2 but they are connected to the insideof the housing 2 via the diaphragms 4 which allow the structures 7 tomove parallel to the plane 30.

It will be understood that once the zero position has been set, theblock 13 b is fixed relative to the housing 2. The screw 60 isstationary relative to the housing because of a tapped portion, and alsothe rod 56 and the head 57, both of which are secured to the blocks 13b, come into abutment against the end of the screw 60.

The diaphragms 4 are flexible along the displacement axis of the rod 3and stiff transversely relative thereto. The structures 7 therefore donot move in any transverse direction.

The diaphragm 4 a thus enables the rod 55 to move, and consequentlyenables the rod 3 to move. The diaphragm 4 a and the diaphragm 4 benable the structures 7 to be moved in translation in order to set thezero position of the system.

FIGS. 6 a and 6 b are diagrams showing possible embodiments for thediaphragms 4 of the invention. It can be seen in these figures that thediaphragms 4 do not necessarily provide the device with sealing.

Sealing can be provided elsewhere by other devices outside the housing2.

In addition, sealing is not necessary in all applications of the deviceof the invention.

The mechanism whereby the structure 7 deforms and the rod 3 is displacedin a device constituting the second embodiment of the invention isdescribed below with reference to FIG. 5 and with reference to FIGS. 7 ato 7 c.

FIG. 5 shows that the elongation axes of the active materials 6 a and 6b are normal to the axes of the rod 3 and of the plates 10 of thestructures 7.

FIGS. 7 a to 7 c thus show three different extension states for theblocks 6 a and 6 b, and consequently three different states for the rod3.

FIG. 7 a shows the initial state or rest position of the device. Theblocks 6 a and 6 b are not powered electrically. Both plates 10 aretherefore parallel to each other and to the plane 30.

The rod 3 is therefore in its zero position, and it is recalled thatthis position can set by acting on the screw 60 which enables thestructures 7 to be moved in translation due to the longitudinalflexibility of the diaphragms 4 a and 4 b.

The initial angle made between the pushers 9 of the two structures 7 ismarked α₀. The initial distance between the blocks 13 a and 13 b isidentified by lines 70.

FIG. 7 b shows that applying electric fields of equal magnitude andopposite sign to the two groups of blocks 6 a and 6 b causes the plates10 to tilt. The two plates 10 remain symmetrical relative to each otherabout the plane 30.

Tilting of the plates 10 has the consequence of changing the anglesbetween the pushers 9.

The pair of pushers 9 connected to the block 13 a forms an angle α₁while the pair of pushers 9 connected to the block 13 b forms an angleα₂. The angles between the pairs of pushers vary with opposite signs,i.e. if α₁ decreases, then α₂ increases, and vice versa, and the amountof variation in each of the angles α₁ and α₂ is different.

Since the block 13 b is fixed relative to the housing 2, varying theangles between the pushers has the consequence of varying the distancebetween the blocks 13 a and 13 b. It can thus be seen that the block 13a is advanced from its initial position as identified by the line 70.The rod 3 is therefore displaced to the left in FIG. 7 b. It is recalledthat displacement of the block 13 a is due to the plates 10 tiltingunder drive from the blocks 6 a and 6 b.

In FIG. 5, arrows 22 show the direction in which the active materials 6a and 6 b lengthen when voltage is applied differently during anotherstage, which stage is also shown in FIG. 7 c. This power supply isopposite to that shown in FIG. 7 b. The plates 10 therefore tilt in theopposite direction and the block 13 a is moved to the right in FIG. 7 c.

The displacement distance of the block 13 a as a function of the appliedvoltage can be evaluated approximately by a geometrical relationshipbetween the three angles α₀, α₁, and α₂. It is also influenced by theelastic deformations that arise in the structure when an external forceis applied.

The rod 3 is movable in translation along the longitudinal axis of thehousing 2. At least one of its ends is connected to a valve in ahydraulic circuit. This makes it possible, for example, to adjust thepressure, or the flow rate of a hydraulic fluid by moving the rod.

By way of example, the device may be used in brake servo-controls or inactuators for flight controls.

Advantageously, the active materials 6 a and 6 b, and the voltagesapplied during the various stages enable the rod 3 to move through astroke having an amplitude of ±0.5 mm.

Advantageously, the force generated by the system and exerted at the endof the rod 3 is equal to or greater than 50 N.

As for the first embodiment, the fact that the directions in which theactive materials 6 a and 6 b lengthen are antisymmetrical relative tothe longitudinal axis 31 of the rod 11 makes it possible to obtainautomatic compensation of the neutral or equilibrium position. Thiscompensation takes place regardless of temperature.

When zero voltage is applied, the rod 3 returns naturally to a stablecentral position. Thus, the system provides good stability for theneutral position.

The deformable structure 7 enables the elongations of the activematerials 6 a and 6 b to be amplified greatly.

In addition, in the second embodiment, the inclination of one of thepairs of pushers 9 is reversed relative to that of the first embodiment.

Furthermore, the displacements of the blocks 13 a and 13 b are decoupledbecause the rod 3 no longer passes through the center of the structures7 and the blocks 13 a and 13 b are no longer both connected to the rod3.

These two characteristics present two main advantages.

Firstly, the deformations at the ends are accumulative so that theresulting displacement is greater.

Secondly, the internal forces of the device are smaller.

In addition, the fact that zero position is simple to adjust by means ofa screw acting on the displacement means via tapping in the housingconstitutes another advantage compared with the first embodiment of thedevice.

The second embodiment is more compact and has a smaller number ofinterfaces in series with the active materials, thereby minimizinglosses in the stiffness of interfaces. This increases the precision withwhich the rod 3 can be moved.

It is also possible to increase the section of the active materials,thus making it possible to increase the stiffness of the device.

The second embodiment may naturally include a counterpressure liquid andexpansion bellows as in the first embodiment.

The first embodiment may also have connection blades instead of portionsof narrow section in the pushers 9. It is thus possible to continuehaving a resilient hinge without play giving flexibility to eachstructure 7.

The device of the invention has numerous advantages.

The means enabling the structure 7 to deform comprise only the portions8 of narrow section. There are no joints. Consequently, there is no wearof parts nor is there any appearance of play as a result of use. Goodaccuracy in displacement is thus obtained, as is a long lifetime.

The active materials 6 a and 6 b are in the form of a solid block whichenables the assembly as a whole to be made very rigid, and also enablesa large force to be delivered by the rod 3.

The valve actuator device thus delivers a large amount of energy for asystem that is compact.

The size of the system as represented by double-headed 32 isadvantageously 50 mm. The size of the device in directions normal to theplane 30 is likewise 50 mm, the portion symmetrical to that shown inFIG. 2 complying within these dimensions. Size is even smaller for thesecond embodiment.

It will be understood from FIG. 3 that the device has at least fourblocks of active material. This redundancy in active material blocksserves further to provide safety in the event of a block of activematerial not functioning correctly. The system can continue to functioneven in the event of one of its blocks breaking down.

Advantageously, the ceramics of the active materials are multi-layermaterials. They thus make it possible to apply low voltages.

1. A device for displacing a moving part along a longitudinal axisrelative to a housing, said device being symmetrical about a planeextending longitudinally relative to the moving part, the devicecomprising two structures that are secured to said moving part andelements of active material comprising at least two blocks extendingbetween inside walls of each structure, with the elongation directionsof the blocks being antisymmetrical about an axis substantiallyperpendicular to the displacement axis of the moving part, wherein thestructures are flexible and suitable for displacing the moving part bydeforming as a function of the elongation of the blocks.
 2. A deviceaccording to claim 1, wherein each structure comprises a plate extendingsubstantially parallel to the displacement axis of the moving part andoblique pushers integral with the plate, at least one pair of pushersbeing secured to the moving part at their ends via at least one junctionblock.
 3. A device according to claim 2, wherein each structure includesportions of narrow section between the plate and the oblique pushers,and between the oblique pushers and the junction blocks connected to themoving part, these portions of narrow section being suitable forimparting flexibility to each structure.
 4. A device according to claim1, wherein each structure comprises a plate extending substantiallyparallel to the displacement axis of the moving part and oblique pushersfitted to said plate, at least one pair of pushers being secured to themoving part at their ends via at least one junction block, theconnection between the plate and the pushers and/or the connectionbetween the pushers and the junction block being a resilient hingewithout play constituted by at least one connecting blade.
 5. A deviceaccording to claim 1, including means suitable for prestressing thedisplacement means between the plates.
 6. A device according to claim 1,wherein the pushers are geometrically antisymmetrical relative to aplane perpendicular to the plates.
 7. A device according to claim 6,wherein one junction block is connected to the moving part and the otherjunction block is fixed relative to the housing external to thestructures, the fixed block being secured to a rod coming into abutmentagainst a screw for adjusting the displacement zero position of themoving part.
 8. A device according to claim 1, wherein the pushers aregeometrically symmetrical about a plane perpendicular to the plates. 9.A device according to claim 8, wherein both junction blocks areconnected to the moving part.
 10. A device according to claim 1,including at least one diaphragm on either side of the junction blocks,the diaphragms being flexible along the displacement axis of the movingpart and stiff transversely relative thereto.
 11. A device according toclaim 10, wherein the diaphragms are suitable for providing sealingbetween the moving part and the housing.
 12. A device according to claim11, wherein the housing is filled with a counterpressure liquid.
 13. Adevice according to claim 10, wherein at least one diaphragm includesbellows.
 14. A device according to claim 10, including at least oneexpansion chamber for the filler liquid.
 15. A device according to claim14, wherein each expansion chamber includes a cutout constituting aportion of the wall of the housing, said cutout being connected to thehousing by bellows.